Plasma treatment is conducted to hydrophilize hydrophobic materials such as plastics, glass, ceramics, metals, semiconductors, etc. For instance, U.S. Pat. No. 5,543,017 discloses a method for plasma-treating a plastic material by glow discharge at an atmospheric pressure, comprising placing a plastic material between a pair of electrodes, at least one of which is insulated, in a plasma reactor; introducing a mixture of (a) argon, an argon-helium mixed gas or an argon-hydrogen mixed gas, and (b) about 18% of saturated steam or a steam-ketone mixed gas at a predetermined temperature into the plasma reactor, and applying a high-frequency voltage to cause glow discharge at an atmospheric pressure, thereby generating plasma.
JP 11-128634 A discloses a method for hydrophilizing a hydrophobic, non-woven fabric without damage, comprising disposing the non-woven fabric between a pair of dielectric-body-covered electrode pipes, and generating plasma between the electrode pipes in a mixed gas atmosphere containing an inert gas.
U.S. Pat. No. 6,399,159 discloses a method for effectively plasma-treating a polyolefin-coated web, comprising (a) supplying a gas to a treatment zone, (b) applying high voltage having a frequency of 1/tc to 2 MHz to a treatment electrode or its opposing electrode, wherein tc is a charging time in which a web surface is exposed to ion current in plasma, to generate plasma in the treatment zone, (c) passing the above web in the treatment zone, and (d) generating reactive species on the polyolefin coating on the above web by the treating gas.
The Research Report No. 3 (2000) of the Tokyo Metropolitan Industrial Technology Research Institute discloses a method for imparting durable hydrophilicity to polyethylene (PE) or polypropylene (PP), comprising placing a sample on a lower one of a pair of electrode plates, causing high-frequency glow discharge in an oxygen atmosphere, treating a plate of PE or PP with oxygen plasma, and treating it with a polar solvent. However, any method of the above references fails to plasma-treat a porous body to its pores.
U.S. Pat. No. 5,403,453 discloses a method of generating durable, uniform glow discharge plasma between a pair of electrodes in a gas kept at an atmospheric pressure, placing a polymer membrane such as a non-woven fabric, etc. in that atmosphere for a certain period of time, forming reactive species while controlling the pressure in a chamber by bellows or a piston, and passing the reactive species through the polymer membrane. Even by this method, however, it is difficult to plasma-treat pores of several μm in a microporous thermoplastic resin membrane.
JP 2003-7497 A discloses an atmospheric-pressure-plasma-treating apparatus comprising a columnar, high-voltage electrode, and a hollow cylindrical ground electrode disposed around the columnar, high-voltage electrode via a cylindrical insulating body, the high-voltage, columnar electrode having pluralities of straight grooves longitudinally extending on its peripheral surface to a tip end of the electrode, and the straight grooves being covered with the cylindrical insulating body to provide gas paths acting as a discharge gap. In this atmospheric-pressure-plasma-treating apparatus, a gas at an atmospheric pressure is introduced into the gas paths, and a high-frequency voltage is applied to both electrodes to generate glow discharge plasma. The resultant plasma gas is ejected from pluralities of exits at the tip end of the electrode (ends of the gas paths), so that only a necessary portion of a work can be treated in a pinpoint manner. However, it is difficult to efficiently plasma-treat the pores of a porous sheet having a large area by this apparatus.